1. Goto resources and search something you want. Save, inventory etc etc.
2. Download the file given.
3. Now here is where it gets fun. Some people have their demos etc as a "myhub" thing instead of a library this is usually the case for things showcasing something and not an actual library of codes. so go to 4.
4. navigate to C>users>yourname>(wherever byond file is) and goto "myhub" or "library". This is where it all gets saved to and you can open them for easy access here if there a myhub file instead of library.
Thats the basics of non libraries.

for libraries.
1. Follow the above steps mostly.
2. Inside your game you cana ctually click a library tab and click the libraries name which will automatically incode the .dm files into your .dme. or you can manually type #include (library name).
3. Read the library documentation its there for a reason and will help you greatly for understanding what it does and how to make it work.